Find the closest element in Binary Search Tree

Given a binary search tree and a target node K. The task is to find the node with minimum absolute difference with given target value K.

Examples:

// For above binary search tree
Input  :  k = 4
Output :  4

Input  :  k = 18
Output :  17

Input  :  k = 12
Output :  9

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

A simple solution for this problem is to store Inorder traversal of given binary search tree in an auxiliary array and then by taking absolute difference of each element find the node having minimum absolute difference with given target value K in linear time.

An efficient solution for this problem is to take advantage of characteristics of BST. Here is the algorithm to solve this problem :

• If target value K is present in given BST, then it’s the node having minimum absolute difference.
• If target value K is less than the value of current node then move to the left child.
• If target value K is greater than the value of current node then move to the right child.
•  // Recursive C++ program to find key closest to k // in given Binary Search Tree. #include using namespace std;    /* A binary tree node has key, pointer to left child and a pointer to right child */ struct Node {     int key;     struct Node* left, *right; };    /* Utility that allocates a new node with the   given key and NULL left and right pointers. */ struct Node* newnode(int key) {     struct Node* node = new (struct Node);     node->key = key;     node->left = node->right  = NULL;     return (node); }    // Function to find node with minimum absolute // difference with given K // min_diff   --> minimum difference till now // min_diff_key  --> node having minimum absolute //                   difference with K void maxDiffUtil(struct Node *ptr, int k, int &min_diff,                                       int &min_diff_key) {     if (ptr == NULL)         return ;        // If k itself is present     if (ptr->key == k)     {         min_diff_key = k;         return;     }        // update min_diff and min_diff_key by checking     // current node value     if (min_diff > abs(ptr->key - k))     {         min_diff = abs(ptr->key - k);         min_diff_key = ptr->key;     }        // if k is less than ptr->key then move in     // left subtree else in right subtree     if (k < ptr->key)         maxDiffUtil(ptr->left, k, min_diff, min_diff_key);     else         maxDiffUtil(ptr->right, k, min_diff, min_diff_key); }    // Wrapper over maxDiffUtil() int maxDiff(Node *root, int k) {     // Initialize minimum difference     int min_diff = INT_MAX, min_diff_key = -1;        // Find value of min_diff_key (Closest key     // in tree with k)     maxDiffUtil(root, k, min_diff, min_diff_key);        return min_diff_key; }    // Driver program to run the case int main() {     struct Node *root = newnode(9);     root->left    = newnode(4);     root->right   = newnode(17);     root->left->left = newnode(3);     root->left->right = newnode(6);     root->left->right->left = newnode(5);     root->left->right->right = newnode(7);     root->right->right = newnode(22);     root->right->right->left = newnode(20);     int k = 18;     cout << maxDiff(root, k);     return 0; }

 // Recursive Java program to find key closest to k // in given Binary Search Tree.     class solution  {              static int min_diff, min_diff_key;          /*  A binary tree node has key, pointer to left child and a pointer to right child */ static class Node {     int key;             Node  left,  right; };     /*  Utility that allocates a new node with the   given key and null left and right pointers.  */     static Node  newnode(int key) {             Node  node = new Node();     node.key = key;     node.left = node.right  = null;     return (node); }     // Function to find node with minimum absolute // difference with given K // min_diff   -. minimum difference till now // min_diff_key  -. node having minimum absolute //                   difference with K static void maxDiffUtil(Node  ptr, int k) {     if (ptr == null)         return ;         // If k itself is present     if (ptr.key == k)     {         min_diff_key = k;         return;     }         // update min_diff and min_diff_key by checking     // current node value     if (min_diff > Math.abs(ptr.key - k))     {         min_diff = Math.abs(ptr.key - k);         min_diff_key = ptr.key;     }         // if k is less than ptr.key then move in     // left subtree else in right subtree     if (k < ptr.key)         maxDiffUtil(ptr.left, k);     else         maxDiffUtil(ptr.right, k); }     // Wrapper over maxDiffUtil() static int maxDiff(Node  root, int k) {     // Initialize minimum difference     min_diff = 999999999; min_diff_key = -1;         // Find value of min_diff_key (Closest key     // in tree with k)     maxDiffUtil(root, k);         return min_diff_key; }     // Driver program to run the case public static void main(String args[]) {             Node  root = newnode(9);     root.left    = newnode(4);     root.right   = newnode(17);     root.left.left = newnode(3);     root.left.right = newnode(6);     root.left.right.left = newnode(5);     root.left.right.right = newnode(7);     root.right.right = newnode(22);     root.right.right.left = newnode(20);     int k = 18;     System.out.println( maxDiff(root, k));        } } //contributed by Arnab Kundu

 # Recursive Python program to find key  # closest to k in given Binary Search Tree.     # Utility that allocates a new node with the  # given key and NULL left and right pointers.  class newnode:         # Constructor to create a new node      def __init__(self, data):          self.key = data          self.left = None         self.right = None    # Function to find node with minimum  # absolute difference with given K  # min_diff --> minimum difference till now  # min_diff_key --> node having minimum absolute  #                   difference with K  def maxDiffUtil(ptr, k, min_diff, min_diff_key):     if ptr == None:          return                # If k itself is present      if ptr.key == k:         min_diff_key = k          return        # update min_diff and min_diff_key by       # checking current node value      if min_diff > abs(ptr.key - k):         min_diff = abs(ptr.key - k)          min_diff_key = ptr.key        # if k is less than ptr->key then move      # in left subtree else in right subtree      if k < ptr.key:         maxDiffUtil(ptr.left, k, min_diff,                                   min_diff_key)     else:         maxDiffUtil(ptr.right, k, min_diff,                                    min_diff_key)    # Wrapper over maxDiffUtil()  def maxDiff(root, k):            # Initialize minimum difference      min_diff, min_diff_key = 999999999999, [-1]        # Find value of min_diff_key (Closest      # key in tree with k)      maxDiffUtil(root, k, min_diff, min_diff_key)        return min_diff_key    # Driver Code if __name__ == '__main__':     root = newnode(9)      root.left = newnode(4)      root.right = newnode(17)     root.left.left = newnode(3)      root.left.right = newnode(6)     root.left.right.left = newnode(5)      root.left.right.right = newnode(7)      root.right.right = newnode(22)     root.right.right.left = newnode(20)      k = 18     print(maxDiff(root, k))    # This code is contributed by PranchalK

 using System;    // Recursive C# program to find key closest to k  // in given Binary Search Tree.      public class solution  {         public static int min_diff, min_diff_key;    /*  A binary tree node has key, pointer to left child  and a pointer to right child */ public class Node {     public int key;         public Node left, right; }    /*  Utility that allocates a new node with the    given key and null left and right pointers.  */    public static Node newnode(int key)  {         Node node = new Node();     node.key = key;     node.left = node.right = null;     return (node);  }    // Function to find node with minimum absolute  // difference with given K  // min_diff   -. minimum difference till now  // min_diff_key  -. node having minimum absolute  //                   difference with K  public static void maxDiffUtil(Node ptr, int k) {     if (ptr == null)     {         return;     }        // If k itself is present      if (ptr.key == k)     {         min_diff_key = k;         return;     }        // update min_diff and min_diff_key by checking      // current node value      if (min_diff > Math.Abs(ptr.key - k))     {         min_diff = Math.Abs(ptr.key - k);         min_diff_key = ptr.key;     }        // if k is less than ptr.key then move in      // left subtree else in right subtree      if (k < ptr.key)     {         maxDiffUtil(ptr.left, k);     }     else     {         maxDiffUtil(ptr.right, k);     } }    // Wrapper over maxDiffUtil()  public static int maxDiff(Node root, int k) {     // Initialize minimum difference      min_diff = 999999999;     min_diff_key = -1;        // Find value of min_diff_key (Closest key      // in tree with k)      maxDiffUtil(root, k);        return min_diff_key; }    // Driver program to run the case  public static void Main(string[] args) {         Node root = newnode(9);     root.left = newnode(4);     root.right = newnode(17);     root.left.left = newnode(3);     root.left.right = newnode(6);     root.left.right.left = newnode(5);     root.left.right.right = newnode(7);     root.right.right = newnode(22);     root.right.right.left = newnode(20);     int k = 18;     Console.WriteLine(maxDiff(root, k));    }  }      // This code is contributed by Shrikant13

Output:
17

Time complexity : O(h) where h is height of given Binary Search Tree.

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